How does stress affect the immune system?

Many research studies have shown that stress is linked to many illnesses. According
to studies stress is also linked with the health of the immune system. Too much
stress and the immune system will become weak, lowering it's defenses to fight off diseases and illnesses. These are called antigens which are foreign substance that induces an immune response in the body such as bacteria, viruses and cancerous cells. This is mainly true for a person with a serious and continuous chronic stress issue.  This suppresses the immune system slowly wearing it away. A weak immune system is more susceptible to infections, illnesses and diseases. Stress also affects the body indirectly when a person may use unhealthy behavioral coping strategies to reduce stress, such as using drugs. 
 

Reference

Daruna, Jorge Hilarion. The Introduction to Psychoneuroimmunology. Boston: Elsevier Academic Press, 2004. Electronic Resource.

Olegario, Janainna and Marcos Silva. Pulmonary Innate Immune Response and
Melatonin Receptors in the Perinatal Stress. Hindawi Publishing, 2013. Print.